The present invention relates to magnetic data storage and retrieval systems. In particular, the present invention relates to measuring head media spacing modulation.
As areal density of magnetic data storage systems continues to increase, it is critical to maintain a controlled spacing between the magnetic recording head and the recording media. The head media spacing modulation is a measure of the displacement of the head relative to the media and is an important aspect of the fly condition of a slider. Head media spacing modulation is often caused by media waviness, disc clamping distortion, write current induced pole tip protrusion, micro-actuator induced modulation, as well as other factors.
Often, it is desired to measure head media spacing to detect, for example, where there is a wave or defect in the media. One such indirect measurement is with the use of a laser doppler vibrometer (LDV). A LDV is mounted on the slider of a disc drive and it measures the slider motion on the backside of a slider. The process essentially first measures head media spacing modulation and disc motion in a vertical direction while the slider is flying at a normal height, and then second, measures the disc vertical motion without flying the slider. The difference between the two measurements is the head media spacing modulation. This indirect measurement cannot measure the actual spacing modulation at the pole tip and cannot measure the spacing change induced by write current.
Another technique used to measure head media spacing modulation is with use of a harmonic ratio. Such a technique to measure head media clearance is described in U.S. Pat. No. 4,777,544, which is incorporated by reference herein. With use of that disclosed method, the harmonic ratio of the readback signal is measured at normal fly height. The slider and head are then lowered to near contact by lowering disc speed, and the harmonic ratio is measured again at near contact. The head media clearance is then calculated at discrete locations based on the two harmonic ratio measurements. This static measurement of head media spacing can give a good measurement of the distance of the head to the media at any particular specified location.
An improved technique for measuring head media modulation is desired.